Hallo,

How much is it important to upload images in high resolution? And why?
How many megapixels make sense? 20? 30? 50? More? Why?
If you want to answer, please add your years of experience, just for our information and if you please!

SS has a 30 megs limit.

Imo it's a bit overkill to submit huge files when they will be used for web-usage 95% of the time + you'll be paid in subs.

Never found low mp images to sell differently in terms of usd per image. Some of my best selling images are even at 4.1 mp, which is practically at shutterstock's 4 mp minimum. In terms of cropping, noise reduction, etc. megapixels give you more options. Storage will be an issue too, especially if you're "trigger happy."

I usually downsize. I dont like to send 36mp files to the micros.  I recall Scott Braut warning against downsizing because of customers who want and will pay for big files.  Unfortunatly the sods are practically non existent thesedays. I cut my 36mp in half usually.

A six MP image will give a decent 10x8 inch print, assuming the image is properly shot (and I've seen one of mine that size enlarged well beyond A4 for use on a roadside advertising board without any problems).  It's a bit cheeky to demand more than that from microstock at the prices people pay.
That said, I've got a lot of 12MP and 20MP images in the mix.
And - to answer your other question - I've been doing this for 13 years and have about 7,000 different images online in one place or another.

My big sale was from an iPhone 5  so who cares anymore.

My old 8mp images I uploaded 10 years ago still sell. I went from 8mp to 12 to 16. I'm still shooting and submitting 16mp images to all the micros but mostly Getty, virtually all get accepted. I keep talking myself out of a new, higher mp camera because it probably won't increase my sales any, not enough to pay for itself.

Some of my best sellers are about 4mp. Some of my worst sellers are closer to 100mp (stitched panos). So obviously there is more than just straight size, Every once in a while I get a large or extra large credit sale on the sites that have them and that is nice. Sadly 90% of my sales seem to be subs these days.

Of course there is nothing to stop a site from dropping the position in the search based on image size.

For SS I generally downsize. I got sick of them rejecting entire batches for out of focus when everyone else took them. Large $ image sales don't seem to happen there anymore, and that might be why - although I did get them 2016 and before.