Inconsistency while uploading images (Flash)

1. Login a as admin
2. Navigate to Images folder
3. Access any folder >> context menu >> add image
4. Select few images from your computer hard drive and click ok

Expected - All the selected images should upload in CMS
Actual - Image is not uploaded at all in the CMS (error or exception is seen) the dialog just disappears and no image is seen

Note
1. Above actual result applies to Chrome & firefox in Windows and Ubuntu.
2. When I disable flash plugin in above browsers I am able to upload the images.
3. In mac with safrai & chrome I am able to upload image with flash plugin on, however of the 7-10 images only 2 or 3 appears in CMS after successful upload (no errors for missing images).