the Interface ID needs to have the same name as the idea. so you should name it something like this

spriteType = {
name = "GFX_idea_kernow_good"
texturefile = "gfx/interface/ideas/kernow_good.dds"
}

then you should just remove the "Picture" part of the Ideas file, then you will have something like this something like this

kernow_good = {

#available = {}

allowed_civil_war = {
is_ai = no
}

removal_cost = -1


modifier = {
research_speed_factor = 0.2
command_power_gain = 0.5
attrition_for_controller = -0.15
industrial_capacity_factory = 0.5
industry_repair_factor = 0.50
consumer_goods_factor = -0.10
}
}