Plenty of platforms enforce two separate limits at once, a maximum file size in kilobytes or megabytes and a maximum pixel dimension, and they rarely mention both in the same error message. A photo compressed to a tidy 1.8MB can still get rejected if it's 6000 pixels wide and the platform caps uploads at 2000 pixels on the long edge.
Compression and resizing solve different problems
Compressing a file shrinks its byte size by discarding redundant data, but it doesn't touch the pixel dimensions at all. If a platform's help page mentions a maximum width or height rather than just a file size, pair this tool with a dimension-based resize, something like resize to 2048x2048 covers a lot of common maximums, so the upload satisfies both requirements instead of just one.
Metadata takes up more room than people expect
Photos straight from a phone or camera often carry EXIF data, GPS coordinates, camera settings and sometimes an embedded thumbnail preview, none of which affects how the image looks but all of which counts toward the file size. Compression strips most of this out automatically, which is part of why a photo can shrink so much without any visible change. You can check what metadata a file is carrying with the image info tool before and after compressing.
iPhone photos need no separate conversion step
HEIC and HEIF files straight from an iPhone can be dropped in directly, this tool decodes them without a separate conversion step first. If a platform specifically rejects HEIC uploads rather than just enforcing a size limit, the HEIC to JPG converter handles that format problem on its own.
When the error message doesn't explain itself
Some upload forms fail silently, spinning forever or simply not saving the image, rather than telling you the file was too big. If a platform's limit isn't stated anywhere, aim for well under 2MB rather than right up against a guess, and test with one photo before batching through a whole folder of uploads.