You just to erase the thing not good on the picture, if the picture only have a tee-shirt and writing, you remove the sentence and just put the logo like a watermark in the middle of the picture and it must be transparent.
All photos must look like the first one or the picture in attachment, without sentences and with a 15% watermark. Some picture have a red cross, just erase the cross and the product on the back and put the watermark.
Sometime the product is in the middle and it's black, like the short just use the logo in white. In fact you are free to choose which white or black logo will look best on the picture.
For the moment I don't have the logo in white so you can do all the picture with the logo in black and I'll try to get my white logo in transparent as soon as possible so you can do the job.