I checked all the images and they had the same file size and the same quality but got replaced anyways and took away 20 points off my upload limit for seemingly no reason.

here's one the examples: https://e621.net/posts/3927820

Yours was: 1280x3232 (505.35 KB, jpg), the replacement was: 1456x3676 (1.3034 MB, png), with the explanation "Free download button / PNG file". The deviantart source has a download button for the fullsize lossless PNG, while the one you uploaded was a smaller lossy JPG.

On the left side of the image in a post is a Replacement link under the History heading, which will take you to a page that shows the replacement image stats compared to the original, along with an explanation. For example: https://e621.net/post_replacements?search%5Bpost_id%5D=3927820

Oh I see.

Now that I took a closer look only the JPG images were the ones that were replaced. The PNGs were left intact.

My mistake.

watsit said:
Yours was: 1280x3232 (505.35 KB, jpg), the replacement was: 1456x3676 (1.3034 MB, png), with the explanation "Free download button / PNG file". The deviantart source has a download button for the fullsize lossless PNG, while the one you uploaded was a smaller lossy JPG.

On the left side of the image in a post is a Replacement link under the History heading, which will take you to a page that shows the replacement image stats compared to the original, along with an explanation. For example: https://e621.net/post_replacements?search%5Bpost_id%5D=3927820

the download button only works if you have ann account I reckon

Because I'm a menace. Note there are also a few that are JPG replaced with JPG, however with same reason as free download button. Because JPGs can fall victim to visual artifacts the files that are in better condition take president and were thusly cross referenced for file condition before being replaced.

versperus said:
Because I'm a menace. Note there are also a few that are JPG replaced with JPG, however with same reason as free download button. Because JPGs can fall victim to visual artifacts the files that are in better condition take president and were thusly cross referenced for file condition before being replaced.

Sometimes this is obvious where even a higher-resolution JPEG file is inferior, but sometimes not that convenient. Some of my recent replacements were like that. A lot of them I got lucky and looking close at text or hair gave the game away for which was worse. Some I got even luckier and had already uploaded the best one. :-D

Needless to say, I double-check sources with Google and Yandex, now.

alphamule said:
Sometimes this is obvious where even a higher-resolution JPEG file is inferior, but sometimes not that convenient. Some of my recent replacements were like that. A lot of them I got lucky and looking close at text or hair gave the game away for which was worse. Some I got even luckier and had already uploaded the best one. :-D

Needless to say, I double-check sources with Google and Yandex, now.

I like to use https://www.diffchecker.com/image-compare/ when I'm unsure of the quality deviation or if there is even one

versperus said:
I like to use https://www.diffchecker.com/image-compare/ when I'm unsure of the quality deviation or if there is even one

WildBit viewer and Irfanview have some nice tools for that. For PNGs, you can just resave two images with identical pixels to BMP and compare them as binary files. Or just resave to same folder with _recompressed on end of filename (and same encoding option). Then do hashes using something like HashCheck.

For JPEGs, you have to look at fine details like hair or text or borders of solid objects. Especially with lower quality setting (like say, 75 versus 95), you'll see noise/spots all over that aren't in the other (likely superior) image.

I should try that tool you linked to.