When you are looking for images to use as stimuli, generally any open-source image search tool works well. You can see some sources I recommend on the favorite tools page, including past stimuli used in the lab.
There are two ways in which I typically edit image stimuli: removing background and re-sizing. I like to edit images with Inkscape, an open-source vector graphics editor. To show how to remove background and resize images in Inkscape, we will edit this picture of doughnuts.
To remove background images, I like to outline the part of the image that I want and use the clip tool. I created a short (silent) YouTube tutorial to show you how to do this. Click the image below to watch it.
Using Inkscape: To resize images with Inkscape, you can create a background of your desired size and export it with your image. I created a (silent) YouTube tutorial for how to do this. It is available by clicking the image below.
Using Preview: It is also reasonably simple to edit images using Preview on a Mac. To begin, open the file you want to edit withPreview. Click and drag around the image to select the area you want.
You can further adjust by clicking and dragging the blue circles. Once you have the selection square the size you want, drag the square to adjust the position. Then you can hit cmd + k to crop the image. Once the image is cropped, hit Tools > Adjust size....
In the Adjust size dialog box, close the lock icon by clicking on it. This locks the aspect ratio to make sure the image does not get distorted. With the lock closed, adjust the size of the image. When the lock is closed, you only need to adjust one dimension (height or width). The other dimension will automatically adjust to preserve the aspect ratio.