These messages refer to the images and other assets used to create the .muse file that you are looking at.
There are 2 messages here. The first one warns that an image has been made larger in Muse than it was originally. Doing this will lower it's visual quality. Since this is usually undesirable, Muse warns when one of these situations exist.
The second message warns that Muse no longer knows the location of the images and other assets used to make the Muse file. This can happen if the folders containing the assets have been moved or renamed, or more likely, because the Muse file was moved/emailed but the images and other assets were not.
The Assets panel will give you some options to resolve these messages. In most situations, these are just messages for your information and not blocking errors. There are a few situations when you will want Muse to know where the assets used to create the .muse file are located.