You should carefully compose and edit the photos you use in your writing, much as you pay attention to the words you use. Advice on how to do this was presented in Images for papers (talk) by David Schroeder on Feb. 1, 2011
The images you use in your writing should meet the following four criteria. They should be
- visually appealing
Images in your writing should not misrepresent truth. If you need to add a parenthetical comment that an image has been edited to communicate more clearly, do so. It is better to have this aside than to be accused of dishonesty.
If your image can not be seen, it can not easily be understood. At its most basic, visibility requires the image be large enough, be bright enough (but not too bright), and not be too blurry. If you have a decent camera, you should be able to capture images that are neither too small nor too blurry. However, getting an image to be bright enough while not too bright can be tricky.
To brighten an image, you should open it up in an image editing program (such as the Gimp or Photoshop) and use the 'Levels' adjustment on your image. You should see a black and white graph, which is a histogram of the brightness of the image. You want to move the white point control to the left until a bit of the graph is to the right of it. As you move the white point control, the image should brighten. Once the image seems bright enough, click on "OK".
You want to communicate with your image, so a reader should be able to extract at least some information without needing to read any text. This is not always possible, but it is a good ideal to strive for.
Finally, images shouldn't look ugly. There is no formulaic way to ensure this happens. Look at the images you make and consider how you like how they look. If possible, have colleagues look at the image and give feedback.