Your "import images" statement is a little vague on details as to what you are trying to do, but the rest of your message suggests that you've worked with Open Street Maps and TransDEM. TransDEM will work with whatever image you've imported into it. But only geo-referenced images will match up with the terrain. TransDEM will alert you if it thinks a raster map is too big. Most of the time a route will use multiple images placed next to each other. TransDEM will try to do as much as it can, being mostly limited by how much memory is available on your computer.
The pink or scarlet arrow that you use to get TranDEM to produce the "Export to Trainz: Basic Files, DEM and Ground Textures" dialog box. There is a section of the dialog that is labelled "Overlay Bitmap (Ground Texture)" and in this section of the dialog is a checkbox "Ground Texture from Raster Map". If that box is checked, TransDEM will imprint the geo-referenced raster map displayed in TransDEM on the terrain.
The imprinted image resolution is not as good as using UTM tiles, but I use it all the time anyway. The resolution will be limited to 10 meter pixels normally, or 5 meter pixels if you are using the Trainz 5 meter grid setting. The grid setting is in the lower left corner of the same dialog box. Shown as "5 m Grid", it allows three settings; "none" (which will cause TransDEM to use the 10 meter pixels), "Route only" (which may use 5 m pixels around the track and 10 m elsewhere), and "All" (which will use 5 m pixels everywhere). I've never used the "Route only" setting, so I'm guessing on that one.
If you are using Open Street Maps in TransDEM and have made the dialog settings suggested, the map should be imprinted on the terrain.
Also, on TransDEM's "Raster Map" menu is "Map Tile Servers". You've probably used that, but if not, you might want to check TransDEM's instructions for enabling and using map tile servers.
Going back to your initial question, I'm not aware of a method of importing images straight into Trainz. Perhaps others are.
