Let's say I have a grassy field. And I want to add several types of flowers to different areas of this field. Using the brush, I can add a layer on top of the image, and paint the area I want.
Can I somehow do this same thing, but using a colored image that repeats when the stroke becomes larger than the image's actual size?
Photoshop's clone tool essentially paints with an image. It will do exactly what you want. This even works if the source image is not part of the final image.
In your example, take your grassy field image and increase the canvas size, say to put several hundred more pixels along the right hand side. Now paste your pictures of flowers into that section.
Now, just clone from flowers onto the grass.
This is similar to what @Phil recommended. Open the image you want as a pattern. Edit that image as you need to, then go Edit > Define Pattern > Name Your Pattern. Then select the Pattern Stamp Tool (under clone stamp tool). Choose your pattern and now you can paint with an image. This tutorial may help and a Google search can find you much more information.
