I came across an answer to this question here: Open multiple images and save them as one ODF or PDF file
However, since it’s an older answer I wanted to provide my own approach that I found more recently. I use LibreOffice Draw to open the images. You can open the images into a page. You can create new pages by right-clicking in the pages section and selecting the new page option. Then you can drag your images into a page and export as pdf.
I recommend using lossless compression to preserve image quality during the export. My use case was combining multiple scanned paystubs into a single pdf with some information redacted. I imported the .png files from my scanner into Draw, redacted the information with rectangles over sensitive areas, and then exported to .pdf with lossless compression. If screenshots of the process would be helpful, let me know and I can follow up with another comment! Fortunately I found the LibreOffice suite to be very intuitive so far. Amazing work by the LibreOffice developers and maintainers.