Edit: cannot upload my oyster pic :(
Is the photo extension .JPG? The lemmy UI does not recognize the capitalized extension as a correct image format. I have changed that for mander, but the change doesn't apply to other instances... Maybe I should submit a pull request for this change if it is a problem for many users. But for now you can try changing the file name to '.jpg' or uploading it to imgur and sharing using that link.