Yes, sometimes screen shots and iPhone photos can be too large. If you don't have a dedicated photo editor like Photoshop, you can open images in Apple's Preview and choose Adjust Size from the Tools menu. I'd suggest that 1200 to 1600 pixels width would be a good size for most images. You can also crop out extraneous parts to save size. Save as .jpg as the compression is better than the .png that screen shots use.
You can also choose Adjust Color if you want to lighten some of the dark areas, not an uncommon need when shooting under the car. There is a shadows slider that works wonders and you can also move the midtone slider under the histogram to open up the midtones, which can also help an image.
Ian