Make a photo collage

Create a photo collage for my pet grooming shop's window poster. Use a square 24x24 inch canvas for print, arrange six customer pet photos in a 3x2 grid with thin white gutters, add a bold header 'Before and After' and a footer with address, phone, and Instagram handle. Ensure all photos are aligned and resolution is print-quality.

Before I print the poster, make the collage instantly scannable: standardize all photos to landscape crops, raise contrast slightly for consistency, add a subtle drop shadow to separate images from the header, and call out any pictures under 300 DPI so I can replace them.

Clients sent a jumble of pet photos for a 24-inch poster and a few are clearly low resolution. I fear the print will look amateur and the shop will be embarrassed. What's the likely quick fix — swap low-res shots, crop tighter, or ask clients for retakes — and which will save time before the Friday print deadline?

Every time I make a poster collage I waste hours adjusting client photos, chasing higher-res images, or redoing crops. Am I failing at intake, file naming, or setting expectations? Recommend one concrete habit to stop late-stage photo headaches and deliver print-ready collages on the first pass.