No need to change path format for windows. Revert it.

Session.vim

@@ -0,0 +1,65 @@
+let SessionLoad = 1
+let s:so_save = &g:so | let s:siso_save = &g:siso | setg so=0 siso=0 | setl so=-1 siso=-1
+let v:this_session=expand("<sfile>:p")
+silent only
+silent tabonly
+cd ~/AppData/Local/nvim-data/lazy/img-paste.vim
+if expand('%') == '' && !&modified && line('$') <= 1 && getline(1) == ''
+  let s:wipebuf = bufnr('%')
+endif
+let s:shortmess_save = &shortmess
+if &shortmess =~ 'A'
+  set shortmess=aoOA
+else
+  set shortmess=aoO
+endif
+badd +240 plugin/mdip.vim
+argglobal
+%argdel
+$argadd ~/AppData/Local/nvim-data/lazy/img-paste.vim
+edit plugin/mdip.vim
+wincmd t
+let s:save_winminheight = &winminheight
+let s:save_winminwidth = &winminwidth
+set winminheight=0
+set winheight=1
+set winminwidth=0
+set winwidth=1
+argglobal
+setlocal fdm=manual
+setlocal fde=0
+setlocal fmr={{{,}}}
+setlocal fdi=#
+setlocal fdl=0
+setlocal fml=1
+setlocal fdn=20
+setlocal fen
+silent! normal! zE
+let &fdl = &fdl
+let s:l = 240 - ((19 * winheight(0) + 13) / 27)
+if s:l < 1 | let s:l = 1 | endif
+keepjumps exe s:l
+normal! zt
+keepjumps 240
+normal! 016|
+tabnext 1
+if exists('s:wipebuf') && len(win_findbuf(s:wipebuf)) == 0 && getbufvar(s:wipebuf, '&buftype') isnot# 'terminal'
+  silent exe 'bwipe ' . s:wipebuf
+endif
+unlet! s:wipebuf
+set winheight=1 winwidth=20
+let &shortmess = s:shortmess_save
+let &winminheight = s:save_winminheight
+let &winminwidth = s:save_winminwidth
+let s:sx = expand("<sfile>:p:r")."x.vim"
+if filereadable(s:sx)
+  exe "source " . fnameescape(s:sx)
+endif
+let &g:so = s:so_save | let &g:siso = s:siso_save
+set hlsearch
+nohlsearch
+let g:this_session = v:this_session
+let g:this_obsession = v:this_session
+doautoall SessionLoadPost
+unlet SessionLoad
+" vim: set ft=vim :

plugin/mdip.vim

@@ -237,8 +237,7 @@ function! mdip#MarkdownClipboardImage()
     else
         " let relpath = s:SaveNewFile(g:mdip_imgdir, tmpfile)
         let extension = split(tmpfile, '\.')[-1]
-        let sep = has('win32') ? '\' : '/'
-        let relpath = g:mdip_imgdir_intext . sep . g:mdip_tmpname . '.' . extension
+        let relpath = g:mdip_imgdir_intext . '/' . g:mdip_tmpname . '.' . extension
         if call(get(g:, 'PasteImageFunction'), [relpath])
             return
         endif