You can resize images in Photoshop very easily.
First you'll need to know what the final size of the decal image should be. You can either measure the area of the model that you are working on to get a size, or measure the original image and divide by the model's scale to come up with the final dimension. Try to make sure the image is a fairly high resolution--300 to 600 dpi should work well. When you're ready to resize, click on the Image dropdown menu and click on Image Size. Here you can directly alter the image size by changing either the pixel count, the dimension size, or the dpi count (or any combination of the above). Make sure that you maintain the dpi count when you resize so that you retain as much detail as possible.
As for cleaning up the image...I've done it both before and after resizing. It depends on the results of the resize and whether certain details need to remain sharp, such as text.
First you'll need to know what the final size of the decal image should be. You can either measure the area of the model that you are working on to get a size, or measure the original image and divide by the model's scale to come up with the final dimension. Try to make sure the image is a fairly high resolution--300 to 600 dpi should work well. When you're ready to resize, click on the Image dropdown menu and click on Image Size. Here you can directly alter the image size by changing either the pixel count, the dimension size, or the dpi count (or any combination of the above). Make sure that you maintain the dpi count when you resize so that you retain as much detail as possible.
As for cleaning up the image...I've done it both before and after resizing. It depends on the results of the resize and whether certain details need to remain sharp, such as text.
