diff --git a/resources/MakeIco.cpp b/resources/MakeIco.cpp
index 19dfdfeb9..c8e69420f 100644
--- a/resources/MakeIco.cpp
+++ b/resources/MakeIco.cpp
@@ -1,67 +1,137 @@
-#include <fstream>
-#include <vector>
 #include <cstdint>
+#include <cstring>
+#include <fstream>
+#include <iostream>
+#include <string>
+#include <vector>
+
+static void writeU32LE(uint8_t *dest, uint32_t value)
+{
+	dest[0] = uint8_t( value        & 0xFF);
+	dest[1] = uint8_t((value >>  8) & 0xFF);
+	dest[2] = uint8_t((value >> 16) & 0xFF);
+	dest[3] = uint8_t((value >> 24) & 0xFF);
+}
+
+static uint32_t readU32BE(const uint8_t *src)
+{
+	return  uint32_t(src[3])        |
+	       (uint32_t(src[2]) <<  8) |
+	       (uint32_t(src[1]) << 16) |
+	       (uint32_t(src[0]) << 24);
+}
 
 int main(int argc, char *argv[])
 {
 	if (argc < 3)
 	{
-		return 1;
-	}
-	auto *outputIcoPath = argv[1];
-	std::ofstream outputIco(outputIcoPath, std::ios::binary);
-	if (!outputIco)
-	{
-		return 2;
+		std::cerr << "usage: " << argv[0] << " OUTPUT INPUT..." << std::endl;
+		exit(1);
 	}
 	auto images = argc - 2;
-	std::vector<char> header(22 + images * 16);
-	auto *incondir = &header[0];
-	*reinterpret_cast<uint16_t *>(&incondir[0]) = 0; // reserved
-	*reinterpret_cast<uint16_t *>(&incondir[2]) = 1; // icon
-	*reinterpret_cast<uint16_t *>(&incondir[4]) = uint16_t(images);
-	std::vector<char> allData;
-	for (auto i = 0; i < images; ++i)
+	if (images > 255)
 	{
-		auto *inputAnyPath = argv[i + 2];
-		std::ifstream inputAny(inputAnyPath, std::ios::binary);
-		std::vector<char> data;
-		while (true)
-		{
-			char ch;
-			inputAny.read(&ch, 1);
-			if (inputAny.eof())
-			{
-				break;
-			}
-			if (!inputAny)
-			{
-				return 3;
-			}
-			data.push_back(ch);
-		}
-		if (*reinterpret_cast<uint64_t *>(&data[0]) != UINT64_C(0x0A1A0A0D474E5089)) // png magic
-		{
-			return 5;
-		}
-		auto width = uint8_t(data[19]);
-		auto height = uint8_t(data[23]);
-		auto *incondirentry = &header[6 + i * 16];
-		*reinterpret_cast<uint8_t *>(&incondirentry[0]) = width;
-		*reinterpret_cast<uint8_t *>(&incondirentry[1]) = height;
-		*reinterpret_cast<uint8_t *>(&incondirentry[2]) = 0; // no color palette
-		*reinterpret_cast<uint8_t *>(&incondirentry[3]) = 0; // reserved
-		*reinterpret_cast<uint16_t *>(&incondirentry[4]) = 1; // 1 color plane
-		*reinterpret_cast<uint16_t *>(&incondirentry[6]) = 32; // 32 bits per pixel
-		*reinterpret_cast<uint32_t *>(&incondirentry[8]) = uint32_t(data.size()); // data size
-		*reinterpret_cast<uint32_t *>(&incondirentry[12]) = uint32_t(header.size() + allData.size()); // data offset
-		allData.insert(allData.end(), data.begin(), data.end());
+		std::cerr << "too many images specified" << std::endl;
+		exit(1);
 	}
-	outputIco.write(&header[0], header.size());
-	outputIco.write(&allData[0], allData.size());
-	if (!outputIco)
+	std::string outputPath = argv[1];
+	std::ofstream output(outputPath, std::ios::binary);
+	auto outputFailure = [&outputPath](std::string action) {
+		std::cerr << "failed to " << action << " " << outputPath << ": " << strerror(errno) << std::endl;
+		exit(1);
+	};
+	if (!output)
 	{
-		return 4;
+		outputFailure("open");
 	}
+	std::vector<char> header(6 + images * 16, 0);
+	auto writeHeader = [&header, &output, &outputFailure]() {
+		output.seekp(0, std::ios_base::beg);
+		output.write(&header[0], header.size());
+		if (!output)
+		{
+			outputFailure("write");
+		}
+	};
+	writeHeader(); // make space for header
+	auto *headerU8 = reinterpret_cast<uint8_t *>(&header[0]);
+	headerU8[2] = 1;
+	headerU8[4] = images;
+	for (auto image = 0; image < images; ++image)
+	{
+		std::string inputPath = argv[2 + image];
+		std::ifstream input(inputPath, std::ios::binary);
+		auto inputFailure = [&inputPath](std::string action) {
+			std::cerr << "failed to " << action << " " << inputPath << ": " << strerror(errno) << std::endl;
+			exit(1);
+		};
+		auto imageFailure = [&inputPath](std::string failure) {
+			std::cerr << "failed to process " << inputPath << ": " << failure << std::endl;
+			exit(1);
+		};
+		if (!input)
+		{
+			inputFailure("open");
+		}
+		std::vector<char> buf;
+		input.seekg(0, std::ios_base::end);
+		buf.resize(input.tellg());
+		input.seekg(0, std::ios_base::beg);
+		input.read(&buf[0], buf.size());
+		if (!input)
+		{
+			inputFailure("read");
+		}
+		auto *bufU8 = reinterpret_cast<uint8_t *>(&buf[0]);
+		if (buf.size() < 0x21 ||
+			readU32BE(&bufU8[0]) != UINT32_C(0x89504E47) ||
+			readU32BE(&bufU8[4]) != UINT32_C(0x0D0A1A0A) ||
+		    bufU8[0x18] != 8 ||
+		    bufU8[0x19] != 6)
+		{
+			imageFailure("not a 32bpp RGBA PNG");
+		}
+		auto writeOffset = output.tellp();
+		output.write(&buf[0], buf.size());
+		if (!output)
+		{
+			outputFailure("write");
+		}
+		auto width = readU32BE(&bufU8[0x10]);
+		auto height = readU32BE(&bufU8[0x14]);
+		if (width == 256)
+		{
+			width = 0;
+		}
+		if (width > 255)
+		{
+			imageFailure("width exceeds U8 limit");
+		}
+		if (height == 256)
+		{
+			height = 0;
+		}
+		if (height > 255)
+		{
+			imageFailure("height exceeds U8 limit");
+		}
+		auto *entryU8 = headerU8 + 6 + image * 16;
+		entryU8[0] = width;
+		entryU8[1] = height;
+		entryU8[4] = 1;
+		entryU8[6] = 32;
+		if (buf.size() > UINT32_MAX)
+		{
+			imageFailure("data size exceeds U32 limit");
+		}
+		writeU32LE(&entryU8[8], uint32_t(buf.size()));
+		if (writeOffset > UINT32_MAX)
+		{
+			std::cerr << "output data size exceeds U32 limit" << std::endl;
+			exit(1);
+		}
+		writeU32LE(&entryU8[12], uint32_t(writeOffset));
+	}
+	writeHeader(); // actually write it out
 	return 0;
 }