This was my 3rd upload from pixiv using an iPad...
My procedures of uploading with an iPad are:
(1)- Go to an image which you think is decent.
(2)- tap the medium sized image, which will produce a big version of the image.
(3)- hold your finger on the image, until 2 options which are 'save image' and 'copy' appear. Select the copy option.
(4)- Go to the web address section of internet browser. (Mine's Safari), and tap twice until the 'paste' option appears. Hit paste and it should appear.
I saved the image onto the Camera Roll of the iPad before uploading. So it is probable that the file's size was shrunk to create less burden for the hard drive...