If you use raster visuals on your poster, they need to be of the right quality. The term we use for this quality, is DPI or PPI (Dots Per Inch/Pixels Per Inch; the last one is used the least, but is actually the correct terminology). The higher the DPI/PPI, the better the quality. For print all images must be of 300 DPI/PPI. For digital media it needs to be 72 DPI/PPI. On this page you can see how to check this in various tools for the images you use on your poster.
Photo robot - by Rock'n Roll Monkey on Unsplash
Adobe Illustrator is a vector based tool, therefor the PPI does not need to be checked. If you imported external raster images in Illustrator, check the PPI in a tool such as Photoshop.