I think the problem is: I want to convert a number of images to a PDF with different page sizes, possibly even within the same document.
Doing that with any software other than Adobe Acrobat that I know of would require multiple template sizes, and if the software allows, multiple page sizes within one document (such as a cover and an interior spread).
The other thing to watch out for is file size and compression quality with the conversion of source document to PDF. Not all PDFs are created equal. For example, I’ve seen examples from the Mac OS and Microsoft Office where the PDF they generate is very poor quality OR very bloated (up to 100x larger than necessary).
I think you can get a trial version of Adobe Acrobat and it might be worth a try. You can create a multipage PDF rom multiple image sources, even if they’re different sizes, with a few clicks in Acrobat and you can control the output resolution and compression.