Originally Posted by 3run
Also, is there any way to set custom 'flag' (check?) image for checkboxes:

You mean custom checkmark? Afaik imgui has no such an option. This is the function which draw checkmarks;

void ImGui::RenderCheckMark(ImVec2 pos, ImU32 col, float sz)
    ImGuiContext& g = *GImGui;
    ImGuiWindow* window = g.CurrentWindow;

    float thickness = ImMax(sz / 5.0f, 1.0f);
    sz -= thickness*0.5f;
    pos += ImVec2(thickness*0.25f, thickness*0.25f);

    float third = sz / 3.0f;
    float bx = pos.x + third;
    float by = pos.y + sz -third * 0.5f;
    window->DrawList->PathLineTo(ImVec2(bx - third, by - third));
    window->DrawList->PathLineTo(ImVec2(bx, by));
    window->DrawList->PathLineTo(ImVec2(bx + third*2, by - third*2));
    window->DrawList->PathStroke(col, false, thickness);

You can change the shape for sure. I don't know if there is a way to load a custom image. Maybe you can try to use "window->DrawList->AddImage" instead of "window->DrawList->PathLineTo" but i'm not sure if this is safe and will work.

in any case, you'll have to create your own dll to do something like this.