Maybe you could elaborate. What was the reason for rejections given?
Are these vectors or just JPGs?
The knife might be better suited as a set. A pic of a knife is nice as a set or you could have it doing something.
The other have potential as clip art. But you have to know why you made those to give it a great keywords and other. Why did you make that house? Was it for purchasing real estate? Know who and why a "buyer" would purchase that. You need to do your research and think of how and why someone would purchase images.
In otherwords don't just make pics, plan on the stuff. Be crafty!