Size limits on covers contributed?

Hello.  I'm new here, glad to find this site, but I have a few questions.  I've searched and searched but could not find any guidelines on the preferred size/resolution of covers submitted.  My main area for now is going to be Famous Monsters Of Filmland/Monster World, and I am painstakingly restoring cover scans to pristine condition, or as close as possible to pristine given the quality of some of the scans available.  The results of my efforts tend to be higher resolution, in lossless PNG format, and while I know that I will need to convert them to JPEG for uploading to your site, I would, of course, prefer not to see my efforts reduced to low quality.  Could someone please advise as to the preferred size/resolution?    
I recently converted all 10 of the Monster World issues to Famous Monsters of Filmland format (it always bothered me about that weird situation with the missing issues 170-179 of FMOF actually being the 10 issues of Monster World, just so Forry could publish his 100th Issue sooner).  I'm don't have Photoshop, just an old Microsoft program that crashes everytime you think about looking at it wrong, and an old underpowered home-built computer, but I did the best I could with what I had.  I'm attaching an example of my efforts, your comments/criticism is welcome.
Also, am I to understand that if I submit a restored FMOF cover, the person who originally submitted the present cover has the right to reject my cover, even if it is a better version?  Maybe I misunderstood that (I hope so).  
Anyhow, I plan to devote a typical miserable Michigan Winter on this project (had our first snow a few days ago).  I'm retired Air Force on disability so I have the time.  
Any advice on this would be much appreciated.
Stephen M Morrison

@hellnbak

As far as resolution goes, the bigger the better, there might be an upper limit on the size of files you can upload but I haven't bumped into it submitting cover art. On the lower end, definitely don't submit thumbnail sized comics. Something around the dimensions in the examples that you posted above is perfect.

If you have a better resolution/better colored/etc version of a cover, by all means upload it, i'm sure the original submitter would have no problem with it. Once you get to 5k points you can set a particular cover as the default image for that issue and remove the other covers. If you want that done before you get to 5k you can either PM a moderator to let them know what you're doing and have them update the series as you progress with your submissions or find a user that has over 5k points thats willing to take the time to help you.   

Picture here can be quite big. Even if you submit 1000x2000 picture for example, it doesn't take much room, 500kb max, at least the one I do. Vary rarely I upload pictures with size of 1mb and over, and those usually happen if I don't check the size before uploading.
 
like that image for example: size is 1518x1977 and it takes 395kb.
http://www.comicvine.com/weave/29-54717/all-images/108-218534/tomb_raider_03/105-1017888/