Depends on the client you are using.
-2009E and older: Simply use the decal/texture options in the insert menu. The client will automatically convert the images you have selected to a Base64 format and bake them into the map file.
-2009M and newer: Open the clients folder inside of your Novetus installation. Open the folder containing the client you are currently using and open the content folder and copy your image into the folder. Next you select the part you want to texture and use the "object" option in the insert menu to insert a decal/texture. When you inserted the decal/texture set the texture/textureID to: rbxasset://<INSERT FILENAME AND EXTENSION HERE> (For example: rbxasset://image01.jpg).