After further digging into this topic I found a way to save the image as a PNG, but it has some limitations.
This solution can be applied to both nodes “Export to Disk” and “Export Render Target”, I’ve tested in Unreal Engine 5.4
I was using a translucent material as a brush to paint on the render target image, but when saving the image was always blank, this was due to the alpha channel, that somehow was always empty, even though my textures had white alpha channels and i also tried to force the alpha channel to be painted white, but it just doesn’t work with a translucent brush.
You need to set the brush material as masked, enable “Used with Editor Compositing” in the material options. The alpha clip value doesn’t have to be 0, if you need a circular brush and not a square one you can make the Alpha clip higher in order to have transparency.
The render target format must be RTF RGBA8 SRGB if you want to mantain the same colors that you see from the camera in game, otherwise use RTF RGBA8.