The first setting to check is whether the compiler is in 'normal mode' rather than 'draft mode' (in the dropdown menu under the Recompile button). This skips some steps to compile quicker, and often doesn't show included images.
Sometimes, there can be problems with the built in PDF viewer, which may cause images to not appear. If everything is correct in the code, the images should still appear in the downloaded PDF file. To resolve this issue, you can simply change the PDF viewer from built in to native using the instructions found here.