Despite the guidelines saying that pngs are an accepted image format, it seems to me that they actually aren't being uploaded properly. I've had this issue before with uploading images but can't find the thread.
When uploading a cover to this submission I first tried a png format, which results in the image not uploading properly (it shows up as a broken chain on the book page and is greyed out on the upload page). I then tried a jpg, and it worked completely fine. Both files are around 3MB so it shouldn't be an issue of size.
Since the png upload doesn't work, I've disabled it and will be uploading jpg for now, but I must have brought up this issue a year or so ago and it evidently didn't get fixed.
Just had a test run on them and didn't have any problems. There can be some problems with images on this site. However, I don't upload many png's and they do seem to have a higher ratio problem rate.
The first time I encountered this was a year ago on this submission and possibly repeated with others. png is my goto format.
I use both pngs and jpgs, and have ran into problems with both formats. Usually waiting for a bit and then reuploading, and/or just switching the format helps.
There are also occasional site slowdowns when it can seem that images are broken or do not get uploaded, but after the slowdown is over, the images do get displayed properly.
I wouldn't chalk it down to a slowdown. The pngs wouldn't display at all, but when I uploaded a jpg right after, it worked immediately. You can also see that the png is still greyed out and "calculating" here and in the other submission I linked
Forgot that editing the images also might fix the issue.
The issue was originally reported here:
I believe staff did some updates to help with upload issues, but they do still happen occasionally.
I just uploaded a png image to https://books.discogs.com/book/205472-a-year-at-avoca-a-cookbook without any problems. I can't say I have ever had any real difficulties uploading png or jpg files to this site, although I have seen a few broken links.