The pdf-forms gem can fill out existing pdf files. This has worked well for me for inserting text into a template PDF.
pdftk supports adding images to an existing pdf via multistamp, which is effectively merging two pdfs together (on top of each other). To add an image to an existing pdf, you could use something like prawn to create a pdf with your image, then use pdftk with multistamp to merge that with your filled out pdf:
pdftk filled_out.pdf multistamp logo.pdf output out.pdf
pdf-forms is a thin wrapper on top of pdftk, so it should allow you to pass the multistamp params required. If not, you can add that functionality or shell out to pdftk directly.
Adobe Acrobat Pro is useful for defining your template fields visually in an existing pdf.