The first setting to check is whether the compiler is in Normal mode rather than Fast [draft] mode (in the drop-down menu under the Recompile button). The Fast [draft] mode skips some steps in order to speed-up compilation, and often doesn't show included images.
Fast-draft-compile.png
Occasionally, issues with the Overleaf PDF viewer may cause images to not appear in the preview—although if everything is correct in the code, the images should still appear in the downloaded PDF file. To resolve this issue, you can change the PDF viewer from Overleaf to Browser using instructions found here.
