You know most pixiv artists are not at all please when people use their work without permission and that does include "drawing" a copy of their work. This is from mochi.◆DF両J-227's profile:
！）無断使用・加工・商用利用禁止 I don't give permission to use the images we own for commercial purposes.
The Japanese translates to "Unauthorized use, editing, and commercial use is prohibited."
At the least if you won't take this down you ought to actually link the original picture to the actual original picture in the artist's gallery and not some reposted image on tumblr: www.pixiv.net/member_illust.ph…