GIMP project file for creating your own close-to-vanilla loading screen textures.
Loading screens are usually 2048 x 1024 pixels.
Take a screenshot of your map, vanilla heists usually take a unique landmark or the area the player spawns in.
Import it into the project into the "INPUT" layer group.
Scale your screenshot up until it's 2048 pixels wide. Move it up or down until everything you want to show is visible.
Play around with the opacity of your screenshot until you can faintly see the background pattern through it, something between 40-50% opacity depending on how bright your screenshot is.
If your screenshot is barely visible, try to brighten it by going to Colors > Brightness-Contrast... and play with the sliders.
When you're happy with it, select the the "EXPORT" layer group all the way at the top and export your image as .dds with BC1 / DXT1 compression.
(If you have empty spaces around your texture after exporting, select the "EXPORT" layer group, go into the "Layer" menu at the top and click on "Layer to Image size". Export it again after that.)