Neverman, some questions:
- What are the pixel resolution(s)?
- Are these the largest size available from the stock sites?
- Would the images be large parts of the signage, or smaller elements inside them?
- Are these all intended for one sign, or different elements for different signs?
- Do you have a mockup?
Dpi isn’t a size measurement; it’s just how large the image would display on a screen vs paper. The important part is the actual amount of pixels in the original images. 6000 x 4000, 2000 x 1500, etc.
Generally speaking, enlarging images beyond 100% resolution degrades the quality. Usually you can get away with enlarging them a bit.
There are some techniques one can use, depending on the final desired output.