Open a layout program (MS Word, MS PPT, Google Slides, etc)
Set up the page with images in order and requested size
(Full, 1/2 page, 1/4 page)
Once images are in the desired layout, save the page as a PDF.
Save each PDF individually
Open Showgram
Create a new page for each PDF you want to add
Name the page
Save the PDF as a PNG
Upload one PNG per page
Save