It's a bug. Let me fix it.
In the meantime, declare the size variable of type UInteger, and it will work.
You can also use SAVE "image" SCREEN$ as in Sinclair BASIC.
EDIT: better try this beta which fixes this bug:
http://www.boriel.com/files/zxb/zxbasic-...a11.tar.gz
http://www.boriel.com/files/zxb/zxbasic-...beta11.zip
http://www.boriel.com/files/zxb/zxbasic-...-win32.zip
http://www.boriel.com/files/zxb/zxbasic-...x64.tar.gz
http://www.boriel.com/files/zxb/zxbasic-...cos.tar.gz
In the meantime, declare the size variable of type UInteger, and it will work.
You can also use SAVE "image" SCREEN$ as in Sinclair BASIC.
EDIT: better try this beta which fixes this bug:
http://www.boriel.com/files/zxb/zxbasic-...a11.tar.gz
http://www.boriel.com/files/zxb/zxbasic-...beta11.zip
http://www.boriel.com/files/zxb/zxbasic-...-win32.zip
http://www.boriel.com/files/zxb/zxbasic-...x64.tar.gz
http://www.boriel.com/files/zxb/zxbasic-...cos.tar.gz