You can try the training program without editing your pictures. This is recommended. Get it working, then worry about editing the pictures.
If everything works the way you want it, great. But the pictures will probably be too big. This the age of digital pictures. You should know how to edit pictures or want to learn. It's a great skill.
The first thing in editing is "crop" the picture. That means to take out the stuff around the edges that you don't want. That makes your picture smaller without losing anything important.
If your picture is still too big, then you have to "resize" it to be smaller. You can judge what fits on your computer. But usually a height of 500 pixels or less is safe.
Widths are trickier, because often there will be two pictures on your computer screen and then it is the combined width that matters. Usually you will be safe with widths less than 340 pixels.
You probably already have a program to edit pictures. Try it. The question is if it does cropping and resizing.
There are lots of picture editors lying around. Picasa (from Google, free) does cropping but apparently not resizing. Irfan is free and does both.
IrfanView (free) is mostly intuitive and mostly works the same as other picture editors. But....How to use it.
Click on "File" at the top, click on "open", and find the picture you want to edit. Draw a square around the part of the picture you want. (Click on one corner of where you want the square. Drag to the opposite corner and release your mouse. You can adjust the sides by clicking on the center and dragging.) Click on edit and then click on "crop selection".
Click on Image and then Resize/Resample. For example if the height is greater than 500, change it to 500. Click on okay. Click on File and then either save to save over your old picture or save as to get a new name.