the brown haired girl is Reimu, It is based on the image zun004.JPG created by ZUN and published in its old blog.
the blonde girl with purple dress is based on a very dark image, She is best known as the girl from Oekaki images, created by Zun
https://en.touhouwiki.net/wiki/Miscellaneous_illustrations_by_ZUN

tremolo_measure said:

the brown haired girl is Reimu, It is based on the image zun004.JPG created by ZUN and published in its old blog.
the blonde girl with purple dress is based on a very dark image, She is best known as the girl from Oekaki images, created by Zun
https://en.touhouwiki.net/wiki/Miscellaneous_illustrations_by_ZUN

Do we know when these Oekaki images have been drawn? Most of his drawings have a date scribbled in the corner, except that for these two it's been cropped out.

MoonMother said:

Do we know when these Oekaki images have been drawn? Most of his drawings have a date scribbled in the corner, except that for these two it's been cropped out.

honestly, i don't know.
I have tried to find the exact source origin but I have not found it, also I searched on several wikis (English, Italian, German Chinese) but none say a specific date.