I've used this site many times for image resizing and image converter without any issues.
From the Authors:
Simple Image Resizer is free online picture resizer
When the size of your photos matters, you often have to give up its quality. But with Simple Image Resizer you don’t have to make this compromise! You can resize your pictures and images without changing their quality. There is no need to install any additional software on your computer to make Simple Image Resizer do its job. You simply browse go to www.simpleimageresizer.com and upload the images you want to shrink. The file formats supported by Simple Image Resizer include JPEG, JPG, PNG, BMP and GIF.
You can use Simple Image Resizer to resize photos and images online, on a variety of different situations, for example:
publishing images on your website to make sure pages load fast
sending images as email attachments
posting photos on blog
posting photos on Facebook, Google+ and other social media
creating internet auction pages, such as eBay, webstore, OnlineAuction, OZtion, WeBidz, uBid, Allegro
posting photos on forum
including pictures within word or pdf documents
Simple Image Resizer helps you reduce picture size without losing quality.