For the most part I use the sync feature of itunes: it allows you to select or create a folder that you can put images into which will automatically be sent to the ipad every time you sync.
You can also use photo stream; it works basically the same as the sync folder except it updates in real-time, it creates a seperate album on your ipad which syncs with a folder on your computer. On PC it will show up in My Computer as Photo Stream (with the ios photo icon). I have had issues with it updating before though.
There is of course drop box as has been mentioned by a couple people here.
If you need to make sure your photos don't get compressed for whatever reason, emailing yourself them and saving them through the mail app is a proven method. Drop box may now support native photo export/import, but I don't know for sure. For the most part compression won't be an issue though.
Hope this helps, I will update my post if I think of anything else.