Forms want one attachment. You have a PDF form and a photo of something. These are different formats, so there is one step in between.
Convert the photo using Images to PDF, selecting A4, Letter or photo-sized pages and suitable margins. Add the resulting images.pdf beside the existing PDF. Check printing when their page sizes differ.
Pages are copied without rasterising their content. Merging does not compress the resulting document.
Check document-level forms, bookmarks, links and signatures separately: rewriting a PDF does not guarantee they survive unchanged.
A photo has to become a PDF page before it can be merged. Use Image to PDF to make a one page PDF from it, choosing A4 for a document like page or the option that fits the page exactly to the photo for no margins. Then add both PDFs to Merge PDF and put them in the order you want.
Check the final page order and count in the list before you merge, because the result cannot be rearranged afterwards. If the photo came out sideways, fix it with Rotate & reorder on the finished file.
No. It combines pages. Raster compression is a separate operation and removes searchable text.
A PDF that needs a password to open must be unlocked first with Protect PDF. Check that you chose the right page, then try again from the original rather than from a half finished copy.
A4 for something that sits alongside a document. Choose the page that fits the photo exactly if you want no white border around it.