Good Morning All
Simply put, not having a thumbnail is an error. Albeit a minor one, it is still an error, and in this case we now show this. Previously, it was a warning, however as we have stated many time warnings will often become errors eventually. Not all will (some are 'subjective' warnings, such as the use of '.lm.txt' lod on lower poly meshes; some assets can only have LOD this way). The DLS has been detecting this particular issue for several years now...
For content on the DLS, the recent DLC clean up pass will have picked up on this error and as such this content is now in the process of being repaired.
Now, the second error you see is when you specify a thumbnail in the config.txt yet the specified file does not exist (e.g. it is in a different folder, or under a different name). This is an error, the file is missing. This does consume resources, as Trainz will attempt to locate the file. The DLS has been detecting this particular error for a few months, so new content won't have this issue. However, there has been a delay between the DLS being updated and Trainz being updated with the most recent error checking. As a note, the error checking won't check for /just/ missing thumbnails. It checks for any image that is referenced in the config.txt but is not present in the asset (e.g. the 'icon' for commodities, the icon for a driver command, images referenced in the string-table for use in the properties menu, etc). A couple of my texture groups referenced non existent images, and threw up the same error...
Again, this is a situation of the content has always been faulty, just Trainz couldn't detect it until now. With the DLS clean up in process, and authors hopefully updating their content to repair the issue, there shouldn't be too long a delay before the issue is resolved. I know I fixed the random scattering of my own items with this (and other missing image errors) last night, and will be releasing them when I've finished a few I don't have installed...
Regards