[ paulc - please feel free to delete this reply in order to keep your thread focusing on your build. Kaz ]
I use direct link to the image file hosted by other app so could be wrong but for jpeg/jpg/png/etc image file, I think you can attach up to 10 files per post.
There are several app allowing you to convert your original image file into the specific file and pixcel size.
I use IrfanView and if using Windows, you can use its PowerToys function Image Resizer.
The limit on the attachment for some of the popular file type;