Images are textures, and textures have LOD while used in game. To avoid any downscaling by the engine, use Texture Group = UI and NoMipMaps. These settings are in the texture details panel.
If this doesn’t solve it, its probably because you are starting from an image that’s too big and the engine cannot scale it down so much without losing detail. So scale it in a gfx program like GIMP and import it again in UE at smaller res.