Picture is just JPG. But like I said, it’s the activity pub message which I believe Lemmy can’t handle very well (or at all?). Its using a so called “attachment” attribute to the message. Here Mastodon also supports it: https://docs.joinmastodon.org/spec/activitypub/#properties-used

Here an example of this:

{"@context":["https:\/\/www.w3.org\/ns\/activitystreams","https:\/\/kbin.melroy.org\/contexts"],"id":"https:\/\/kbin.melroy.org\/m\/fediverse@lemmy.world\/t\/506689\/-\/comment\/4458544","type":"Note","attributedTo":"https:\/\/kbin.melroy.org\/u\/melroy","inReplyTo":"https:\/\/lemmy.ca\/post\/30865604","to":["https:\/\/www.w3.org\/ns\/activitystreams#Public","https:\/\/lemmy.ca\/u\/otter"],"cc":["https:\/\/lemmy.world\/c\/fediverse","https:\/\/kbin.melroy.org\/u\/melroy\/followers"],"sensitive":false,"content":"\u003Cp\u003EHere is the image.\u003C\/p\u003E\n","mediaType":"text\/html","source":{"content":"Here is the image.","mediaType":"text\/markdown"},"url":"https:\/\/kbin.melroy.org\/m\/fediverse@lemmy.world\/t\/506689\/-\/comment\/4458544","tag":[{"type":"Mention","href":"https:\/\/lemmy.ca\/u\/otter","name":"@otter@lemmy.ca"}],"published":"2024-10-13T14:36:34+02:00","contentMap":{"en":"\u003Cp\u003EHere is the image.\u003C\/p\u003E\n"},"attachment":[{"type":"Image","mediaType":"image\/jpeg","url":"https:\/\/kbin.melroy.org\/media\/12\/4f\/124f315062cb3fd1d8ae68e43c50805c64cf4d0b5ad5935837f9f9a1b55a15df.jpg","name":"Tips \u0026 Tricks to quit twitter","blurhash":"LdQ,2c~CS4-pMd$%R+af$MMyofRj","focalPoint":[0,0],"width":2480,"height":3354}],"image":{"type":"Image","url":"https:\/\/kbin.melroy.org\/media\/12\/4f\/124f315062cb3fd1d8ae68e43c50805c64cf4d0b5ad5935837f9f9a1b55a15df.jpg"}}

Indeed, I asked Melroy above which format they use