If you mean software then photoshop if you want to buy it or paint dot net for free is also brilliant.
If you mean base then if you have a careful look through this forum or the Irish Rail FOI requests you can find technical drawings for the entire fleet in terms of liveries. Then you will have the shape with the proper proportions for the loco/stock you want.
If you want to get into editing photos then there is plenty of youtube tutorials on editing photographs , some of the guys who do the wacky car photo edits would be of help like this.