I recommend:

https :// jpegresizer.com/

It compresses image files further without resizing the dimension of the picture.
The output quality is still good. I use it to further compress 3~4MB JPEG (from Android Phone camera app) to below 1MB (and at the same time maintain the same height and width), so that you can upload using forum's Upload Images function.

I use simpleimageresizer. You may resize your image either by Percentage or by Dimensions.

They are such as great image resizer but I'm recommending online application such as Compress Jpeg you can optimize your images without losing image quality.