Hello, hope you're having a good time.
So I work with a charity club (for cancer kids and disabled kids and people with aids) and some celebrities and artists gave us some original paintings that they have painted themselves and we want to sell them for fundraising for the cause.

Can you please tell me how and where I must put them on sale?
Also please note that we are stationed in Iran and I THINK it would be a little hard to send them worldwide I'm not sure.

Thanks for your time.

People sell paintings worldwide on ebay and etsy. There are fine art sites too, but I would start with ebay and etsy.
If your charity is registered as an official not for profit, the fees are much lower. (I'm not sure about out of the US, however.)

There are 3 problems with this:
The internet is overloaded with art for sale.
The viewer can't see the paintings in person, to get a sense of color and depth and all that goes into painting.
Shipping is indeed expensive, especially when frames, glass, and large size are involved. Insurance is another problem, because it isn't often possible to assign a value.

I wouldn't try, personally, speaking as a long time ebay seller.
Charity events do better when arranged locally.
And what you will save on shipping and fees will more than cover the cost of advertising. They will sell better anyway.

Maybe this site can be of interest and give you some ideas.

Iranians Can Now Buy from Amazon & eBay. Kind of. (http://techrasa.com/2015/09/08/iranians-can-now-buy-amazon-ebay-kind/)