Essentially anything that will tell you story. Common images found on posters are -
Sure can! You can use your own photographs, drawings, graphics, etc., as long as you give yourself credit.
You can also use charts you create - Those don't necessarily need a citation that you made it because it is your actual research data.
Absolutely! All images must be cited.
The only instances when you don't have to cite images is -
We recommend the sources below because they are royalty-free images and images with special open licensing that allow reuse with or without crediting the creator. There are plenty of royalty-free websites available besides the ones we list here.
We don't recommend Google Images because not everything on there is allowed to be reused.
Captions are appropriate! They help readers understand what you are depicting. The placement of your caption is most important. Caption placement typically is above or below image. Sometimes, you will find it on the left- or right-side. Occasionally, you will find caption on top of images.
If you do place a caption on an image, make sure it is legible (see the example image below).
Chen, A. (2019 Oct 12). The controversy of accessible type + download a pocket guide of best practices. Medium, https://medium.com/queer-design-club/the-controversy-of-accessible-type-8def04eb8808