Image and Art posts can, on occasion, throw an error telling you that one or more of the images you uploaded failed to convert properly.
This happens when the image handling server tries to process something that it thinks isn't an image, and tells the part of the site that handles posts that it can't do anything with it. Luckily, there's a few things you can try to make it go through next time.
Check your file formats
While the image handling suite is fairly robust and capable of handling most image formats, some more exotic ones can cause problems, and the site tries to stop you uploading these even if they technically work.
The three file formats that are safe bets are the standard web trio. In order of most to least ideal, these are PNG, JPEG, and GIF.
PNG files have the best chance of looking great after the conversion, and is the format most recommended for Art posts.
JPEG is also a good option, and is famous for having small file sizes. However, the compression the format uses means a lot of detail is lost, making them kind of ugly even for regular image posts. Detail lost in a JPEG can't be restored by our algorithm, unfortunately.
GIFs are a fine option for animations.
If that didn't work...
... then chances are it's a problem on the site's end. All image conversion failures are logged, so if there's a technical problem it can be looked into. However, it's still a good idea to create a ticket to give a heads up so you can be pinged when it's fixed.