Presumably if you're looking at a gtkrc file then it's GTK+2 and not GTK+3 (which uses CSS for themes)
Here's any example from one of my GIMP themes that styles checkboxes:
style "checkbuttons" = "gimp-default-style"
{
engine "pixmap"
{
image
{
function = CHECK
state = NORMAL
shadow = OUT
overlay_file = "/checkbuttons/check-norm-unchecked.png" # image for unchecked check button
overlay_stretch = TRUE # set TRUE to get GtkCheckButton::indicator-size to be used
}
image
{
function = CHECK
state = PRELIGHT
shadow = OUT
overlay_file = "/checkbuttons/check-prelight-unchecked.png" # image for hovered-over unchecked check button
overlay_stretch = TRUE
}
image
{
function = CHECK
state = ACTIVE
shadow = OUT
overlay_file = "/checkbuttons/check-norm-unchecked.png" # image for unchecked check button when being clicked
overlay_stretch = TRUE
}
image
{
function = CHECK
state = INSENSITIVE
shadow = OUT
overlay_file = "/checkbuttons/check-insens-unchecked.png" # image for unchecked disabled check button
overlay_stretch = TRUE
}
image
{
function = CHECK
recolorable = TRUE
state = NORMAL
shadow = IN
overlay_file = "/checkbuttons/check-norm-checked.png" # image for checked check button
overlay_stretch = TRUE
}
image
{
function = CHECK
recolorable = TRUE
state = PRELIGHT
shadow = IN
overlay_file = "/checkbuttons/check-prelight-checked.png" # image for hovered-over checked check button
overlay_stretch = TRUE
}
image
{
function = CHECK
recolorable = TRUE
state = ACTIVE
shadow = IN
overlay_file = "/checkbuttons/check-prelight-checked.png" # image for clicked checked check button
overlay_stretch = TRUE
}
image
{
function = CHECK
recolorable = TRUE
state = INSENSITIVE
shadow = IN
overlay_file = "/checkbuttons/check-insens-checked.png" # image for disabled checked check button
overlay_stretch = TRUE
}
image
{
function = FLAT_BOX
recolorable = TRUE
file = "/checkbuttons/checklight.png" # Used to create a highlight for the whole widget (including label)
border = { 1, 1, 1, 1}
stretch = TRUE
}
}
}
And for radiobuttons:
style "radiobuttons" = "gimp-default-style"
{
engine "pixmap"
{
image
{
function = OPTION
state = NORMAL
shadow = OUT
overlay_file = "/radiobuttons/radio-normal-unchecked.png" # image for unchecked radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
state = PRELIGHT
shadow = OUT
overlay_file = "/radiobuttons/radio-prelight-unchecked.png" # image for hovered-over unchecked radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
state = ACTIVE
shadow = OUT
overlay_file = "/radiobuttons/radio-normal-unchecked.png" # image for unchecked radio button when being clicked
overlay_stretch = TRUE
}
image
{
function = OPTION
state = INSENSITIVE
shadow = OUT
overlay_file = "/radiobuttons/radio-insens-unchecked.png" # image for disabled radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
recolorable = TRUE
state = NORMAL
shadow = IN
overlay_file = "/radiobuttons/radio-norm-checked.png" # image for checked radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
recolorable = TRUE
state = PRELIGHT
shadow = IN
overlay_file = "/radiobuttons/radio-prelight-checked.png" # image for hovered-over checked radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
recolorable = TRUE
state = ACTIVE
shadow = IN
overlay_file = "/radiobuttons/radio-prelight-checked.png" # image for clicked checked radio button
overlay_stretch = TRUE
}
image
{
function = OPTION
recolorable = TRUE
state = INSENSITIVE
shadow = IN
overlay_file = "/radiobuttons/radio-insens-checked.png" # image for disabled checked radio button
overlay_stretch = TRUE
}
image
{
function = FLAT_BOX
recolorable = TRUE
file = "/radiobuttons/radiolight.png" # Used to create a highlight for the whole widget (including label)
border = { 1, 1, 1, 1}
stretch = TRUE
}
}
}
class "GtkCheckButton" style "checkbuttons"
class "GtkCheckMenuItem" style "checkbuttons"
class "GtkRadioButton" style "radiobuttons"
class "GtkRadioMenuItem" style "radiobuttons"
This might help you in examining the gtkrc file to find where it's getting the image files from.
Kevin