Do the practice welds as suggested. Come on now MIG welding is pretty easy. Just get some similar metal, put it on a bench and weld it together. By the look of your welds you are seriously under penetrating. So this is how I like to do things, over due it. Turn it up as hot as you can and melt the damn thing thru, blow a hole to two, get a feeling of what is TOO MUCH, then back it down and practice. Weld something together and then clamp it in a vice and with a big hammer beat the crap out of it to try to break your weld. This is a common "destructive test" and it should be done on anything being welded where there is any doubt. And Ant, you should have some serious doubts with those welds!
But all you need is some practice, don't touch that frame until you burn a few holes in test metal and beat it up with a hammer.